Vehicle headlights are primarily used to provide frontal illumination for improved visibility during nighttime driving. During use, most vehicle headlights have at least two states of operation. One is the high beam state used while driving on roadways without the presence of preceding or on-coming traffic. Another is the low beam state used during most nighttime driving conditions so as to enable the driver to see the roadway ahead while limiting dazzling glare that would otherwise result from use of the high beam state.
For low beam operation, governmental regulations typically require that the vehicle headlight provide a light distribution beam pattern having specific photometric requirements intended to protect on-coming traffic from harmful glare. Projector lamp headlights meeting these requirements typically exhibit a relatively stark light/dark cutoff line in the focused beam pattern. This sharp, so-called light-shadow line results from the use of an opaque metal mask to limit upward beam projection. The mask is positioned in the lower portion of the light path between the light source (and/or reflector) and the projector lens. Because of the inverting properties of the typical projector lens, the shadow cast by the mask is transferred to the upper regions of the projected beam pattern. Thus, the top edge of the mask creates, in the projected beam pattern from the headlight assembly, the light-shadow line in which the light is below the line and the shadow is above the line. This dark above/bright below distribution pattern provides lighting for road surface visibility, yet attempts to minimize glare to oncoming traffic.
When an automobile encounters a bump or dip in the roadway, the projected beam pattern can temporarily rise into the view of on-coming traffic. Because of the sharp line-shadow boundary, on-coming traffic will perceive the rising and falling beam pattern as a distracting flash of light.
Furthermore, projector-style headlights with metallic masks can create a so-called “chromatic aberration” or undesirable chromatic effect which tends to bring different colors of light into focus at different points. This aberration yields an effect of bright colored light bands that may annoy or confuse on-coming drivers. The colored light bands are believed to be created by the top edge of the metallic masks, although the underlying mechanism which creates this effect is disputed. According to one theory, the white light emanating from the light source strikes the top edge of the metallic mask, creating a prismatic lensing affect with the air around the top edge, thereby splitting the white light into the colored light bands (as depicted in FIG. 6A). According to another theory, light reflecting at an oblique angle off the top edge of the metallic mask is polarized and one orientation of the visible electromagnetic field vector is more prevalent (as depicted in FIG. 6B). Regardless of the cause, chromatic aberration is known to result in an undesirable effect in which the colored light bands often appear blue in color and are most noticeable when an on-coming vehicle having projector-style headlights hits a bump or dip in the road causing brief flashes of blue-violet light. These flashes impose distracting glare on opposing traffic, and may confuse an on-coming driver to think the approaching vehicle is a police car or other emergency vehicle.
There exists, therefore, a need to reduce the negative glare effects of the sharp light-shadow boundary as well as those caused by chromatic aberration, for the benefit of on-coming traffic.